This invention relates to an electronic balance incorporating an improved Roberval mechanism, having at one end thereof a weighing pan attached through a movable vertical arm maintained in a vertical position and at the other end thereof a shutter coupled with a position sensor.
The principles of an electronic balance incorporating a Roberval mechanism will be explained first with reference to FIG. 4, wherein numeral 2 indicates a lever beam of the balance supported at a supporting point 4 in the middle. A weighing pan 8 for carrying an object to be weighed is attached to one end of the beam 2 through a vertical arm 6 which is adapted to move while maintaining a vertical orientation. A shutter 16, adapted to couple with a position sensor 18, is provided at the other end of the beam 2. The aforementioned Roberval mechanism is for the purpose of moving the vertically extended arm 6 while maintaining its vertical orientation and is comprised of a pair of mutually parallel horizontal arms 12 and 14, one end of which is respectively connected rotatably to the top end and the bottom end of the vertical arm 6, while the other ends of these horizontal arms 12 and 14 are rotatably supported by a fixed part such as the main body of the balance. The position sensor 18 is fastened also to a fixed part such as the main body of the balance. The Roberval mechanism has the known advantage of causing the same force to be applied to the balance, independently of the position of a sample 10 placed on the weighing pan 8.
With the electronic balance shown in FIG. 4, the shutter 16 moves vertically according to the movement of the vertical arm 6, and it is this motion of the shutter 16 that is detected by the position sensor 18. To improve the sensitivity of such a balance, or to increase the output of such a position sensor when a sample is placed on the weighing pan 8, it is necessary to make the beam 2 longer so as to improve its angular resolution. If the beam 2 is made longer, however, it affects the overall size of the electronic balance. It is an object of the present invention to improve the sensitivity of an electronic balance without increasing the length of its beam.